What does the term "sucking up" mean?

"Sucking up" is the English vernacular phrase that means to act as what a toady or a sycophant would. In other words, one who sucks up to another person agrees with that person to a degree that the first person never disagrees with the second person. The person who sucks up usually does so to gain an advantage.

A person is "sucking up" to another when they just do and/or say things to the other person so that the other person is fond of him/her and may treat him/her better. When someone "sucks up" to another person, they generally overpraise and do favors for the other person that would not normally be done.